This invention relates to a magnetic recording medium for use as video tape, audio tape, magnetic disk or the like and a process for manufacturing the same. More particularly, the invention concerns a magnetic recording medium with various properties modified through improved dispersion of a magnetic powder, and where necessary of an abrasive too, in the magnetic layer, and also concerns a process for manufacturing the medium.
Heretofore, the magnetic recording medium such as magnetic tape has been commonly manufactured by dispersing and mixing a magnetic iron oxide or ferromagnetic powder in the form of needles or granules together with a dispersing agent, binder, lubricant, with or without the addition of other ingredients on a ball mill, applying the resulting mixture to a base film of polyester or the like, and then drying the coat to form a magnetic layer over the base.
The recent tendency toward higher density recording has caused a strong demand for magnetic recording media with better signal-to-noise ratio and improved surface properties. To satisfy the demand varied attempts have already been made. Typical of them is the addition of a dispersing agent aimed at improving the dispersibility of the magnetic powder and other additives in the binder. Usually, such a dispersing agent is simply added to a mixture of the magnetic powder, additives, and binder, and the components are dispersed and mixed altogether. The dispersant addition in this manner does not produce a good dispersion because the solid particles can scarcely be dispersed. It therefore brings only slight improvements in S/N ratio and other recording medium characteristics.
The dispersing agents for this application have been limited to ordinary surface active agents.
In view of the foregoing, intensive research has been made to obtain a magnetic recording medium with properties modified through improved dispersion of the magnetic powder and additives into the binder and also to broaden the applicable scope of the dispersibility improver. As a result, it has now been found that a very good dispersion is attained by allowing the magnetic powder, and where necessary the abrasive too when the latter is employed, to adsorb a polycarboxylic acid copolymer in advance and then mixing the magnetic powder, and also the abrasive where necessary, thus treated on the surface with the binder. The discovery has led to the present invention.